There is an ever present need for higher density information storage media for computers. Currently, the prevalent storage media is the hard disk drive (HDD). An HDD is a non-volatile storage device which stores digitally encoded data on rapidly rotating disks with magnetic surfaces. The disks are circular, with a central hole. The disks are made from a non-magnetic material, usually glass or aluminum, and are coated on one or both sides with magnetic thin films, such as cobalt-based alloy thin films. HDDs record data by magnetizing regions of the magnetic film with one of two particular orientations, allowing binary data storage in the film. The stored data is read by detecting the orientation of the magnetized regions of the film.
A typical HDD design consists of a spindle which holds one or more disks, spaced sufficiently apart to allow read-write heads to access one or both sides of one or more disks. The disks are fixed to the spindle by clamps inserted into the central holes in the disks. The disks are spun at very high speeds. Information is written onto and read off a disk as it rotates past the read-write heads. The heads move in very close proximity to the surface of the magnetic thin film. The read-write head is used to detect and/or modify the magnetization of the material immediately underneath it. There is one head for each magnetic disk surface on the spindle. An arm moves the heads across the spinning disks, allowing each head to access almost the entire surface of a corresponding disk.
In a conventional magnetic media, each bit cell includes a plurality of magnetic grains randomly dispersed. Ideally, the plurality of magnetic grains are physically separated from each other so as to provide improved write-ability, signal to noise ratio (SNR) and thermal stability.
As the aerial density of magnetic recording media increases, number of bit cells per square inch increases. This reduces the size of the bit cell. To effectively measure a transition, a minimum number of magnetic grains are required in a bit cell. As the size of a bit cell reduces, the magnetic grain size has to be correspondingly reduced to provide a minimum number of magnetic grains in the bit cell. If isolation of magnetic grains and reduction in magnetic grain size are advanced to ensure low noise, the recording density will be limited because of thermal disturbances.
For improvement of a recording density, it is desirable to reduce a recording cell size on a media, which brings about reduction in signal magnetic field intensity generated from the media. In order to meet the SNR required for a recording system, noise must be reduced corresponding to reduction in signal intensity. The media noise is mainly caused by fluctuation of a magnetization transition, and the fluctuation is proportional to a size of a magnetization reversal unit made of magnetic grains. Therefore, in order to reduce the media noise, it is required to isolate magnetic grains by disrupting exchange interaction between magnetic grains.
Magnetic energy of a single isolated magnetic grain is given by a product of magnetic anisotropy energy density and volume of the grain. It is desirable to reduce the media thickness in order to reduce a magnetization transition width. It is also desirable to reduce the grain size in order to meet a requirement for low noise. Reduced magnetic grain size significantly lowers the volume of magnetic grain, and further significantly lowers magnetic energy of the grain. If the magnetic energy of a given magnetic grain in a magnetic media is several hundred times the thermal energy at an operating temperature (for example, at room temperature), resistance against thermal disturbance is considered to be sufficient. However, if the magnetic energy of the magnetic grain is less than a hundred times the thermal energy, there is a possibility that the magnetization direction of the magnetic grain may be reversed by thermal disturbance, potentially leading to loss of recorded information.
Various alternatives have been proposed to overcome the problem of thermal disturbances. One alternative is to use a magnetic material with high magnetic anisotropy. These magnetic materials need higher recording saturation magnetic field from a head to write the magnetic media. Another alternative is to use thermally assisted recording, where a highly anisotropic magnetic material is used and a recording portion is heated by light irradiation during recording. The heat lowers the anisotropy of magnetic grains and the recording saturation magnetic field. This permits writing of the magnetic media with conventional magnetic head.
As the aerial density increases, there are a minimum number of magnetic grains that are still required per bit cell and there is a limitation on how small a magnetic grain can be practically achieved.
An alternate magnetic media that is being explored is a patterned media, where magnetic portions alternate with non-magnetic portions. For example, a bit patterned media may have magnetic portions defining a magnetic domain as islands surrounded by non-magnetic portions. A track patterned media may have for example, a concentric track of magnetic portions separated by non-magnetic portions.
Various alternatives have been proposed to manufacture these media, however there still remains a need to come up with a method that is cost effective and compatible with high volume manufacturing. It is in this context that the embodiments of this disclosure arise.